Lawnmowers traditionally come in two varieties: rotary mowers, where blades rotate horizontally in a plane; and reel mowers, where helical blades rotate about an axis so that the helical blades can shear grass against a bedknife.
Rotary mowers have blades with a small cutting area, and the blades require very fast tip speeds to perform well. Typical tip speeds are greater than fifteen thousand feet per minute. This is necessary to achieve an acceptable cut, even when blades are sharp. Foreign objects struck by blades at these velocities can be very dangerous. Even though rotary mower decks and collection bags are required to be designed in accordance with various safety standards to minimize the risk of injury due to flying objects, many injuries still occur from ejected objects even when safety measures are in place.
In addition, many injuries occur from the blades themselves. Rotary mower blades are very large and heavy and will inflict grievous injury even at low speeds. Furthermore, decks for these blades are completely open underneath to accommodate the circular cut area and the need for the blades to cut at their tips. This large, open area increases the risk of foot, hand, or other body part mutilation, and increases the risk of the blades striking a foreign object.
Reel mowers have multiple helical blades (usually five or more) that rotate about a horizontal shaft; a stationary bedknife provides a shearing surface for the helical blades. The rotating helical blades of a reel mower typically operate at a lower speed than the blades of a rotary mower, but reel mowers are precision instruments that require frequent adjustment and precise operating conditions such as rotational speed and forward velocity. Reel mowers are also dangerous. Reel mowers must expose the entire front of the rotating helical blades to allow the blades to feed grass against a bedknife. Even while not in operation the helical blades may cause injury. In operation, accidental contact with a spinning helical blade will pull a hand or foot into the bedknife. Consequently, there is a need for a mower with a shielded and less exposed blade assembly to reduce the risk of injury.
Typical rotary lawnmower blades are generally heavy, flat elongated pieces of steel that rotate symmetrically about a rotatable vertical shaft. Rotary lawn mower blades have a sharpened leading edge, and the outer portion of the trailing edge is curved to create airflow to lift grass and blow clippings into a bag or out of a discharge passage.